This invention relates to inhibiting catalyst coke formation in the manufacture of an olefin. In particular, this invention relates to inhibiting coke formation on a silicoaluminophosphate molecular sieve catalyst in the manufacture of ethylene and/or propylene.
Demand for polyolefins, e.g., polyethylene and polypropylene, has been steadily increasing. It is projected that the increased demand for polyolefins will outpace the availability of raw materials, e.g., ethylene and propylene, from which polyolefins can be made.
Olefins which are used to make polyolefins have been traditionally produced from petroleum feedstock by either catalytic or steam cracking of the petroleum. The cost of petroleum cracking has steadily increased, however, making it important to find alternative feedstock sources for olefins.
Oxygenates are a promising alternative feedstock for making olefins. Particularly promising oxygenate feedstock are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials such as coal, recycled plastics, municipal wastes, or any appropriate organic material. Because of the wide variety of sources, oxygenates have promise as an economical source for olefin production.
One way in which olefins can be made from the alternative oxygenate feedstock is by catalytic conversion. In U.S. Pat. No. 4,499,327, for example, a catalytic process for converting methanol to olefins is described. The catalyst used in that process contains a silicoaluminophosphate molecular sieve.
Of course, it is highly desirable to convert as much of the oxygenate feedstock as possible into as much olefin product as possible. Various methods of doing such have been suggested. For example, U.S. Pat. No. 4,677,242 describes a method of increasing the amount of ethylene and propylene produced from the catalytic conversion of methanol by adding an aromatic diluent to the methanol. The catalyst that is used in the process contains a silicoaluminophosphate molecular sieve. The use of the diluent is considered to result in an increased amount of ethylene product.
U.S. Pat. No. 4,499,314 also discloses a catalytic process for converting methanol to ethylene and para-xylene. The catalyst that is used is ZSM-5. Promoters are used to promote either the formation of aromatics products or olefins products. Benzene, toluene and para-xylene are preferred aromatics promoters. Ethylene, propylene and butenes are preferred olefin promoters.
Song et al., xe2x80x9cSupramolecular Origins of Product Selectivity for Methanol-to Olefin Catalysis on HSAPO-34,xe2x80x9d J. Am. Chem. Soc., 2001, 123, pp. 4749-4754, indicate that ethylene selectivity in methanol-to-olefin (MTO) catalysis is related to the number of methyl groups on benzene rings trapped in the cages of a preferred HSAPO-34 catalyst. Co-feeding water with methanol was found to significantly increase the average number of methyl groups per ring, and increase ethylene selectivity.
U.S. Pat. No. 6,166,282, discloses an MTO process which uses a fast-fluidized bed reactor. The process is carried out in a reaction zone having a dense phase zone in the lower reaction zone and a transition zone which extends into a catalyst disengaging zone. A portion of the catalyst is circulated from the disengaging zone to the lower, dense phase zone. The arrangement is considered to enable reduction in catalyst inventory.
There remains, nevertheless, a desire to improve the economic attractiveness of the oxygenate conversion process. Catalysts and methods to produce olefins from oxygenates are needed which increase the selectivity of the oxygenate conversion reaction, particularly to ethylene and propylene, without resorting to adding costly product enhancing promoters.
In order to overcome the various problems associated with providing large quantities of olefin product which can ultimately be used in the manufacture of polyolefin compositions, this invention provides a method of inhibiting catalyst coke formation during the conversion of an oxygenate feedstock to an olefin-containing product. In a preferred embodiment, the oxygenate feedstock is reacted with a silicoaluminophosphate molecular sieve catalyst at an average reactor temperature effective to form the olefin-containing product. The olefin-containing product is then separated from the catalyst and at least a portion of the catalyst is cooled to a temperature below the average reactor temperature. The cooled portion of the catalyst is then contacted with oxygenate-containing feedstock without first regenerating the catalyst.
It is preferred that the average reactor temperature of this method is from of about 350xc2x0 C. to about 525xc2x0 C. and the cooled portion of the catalyst is from about 10xc2x0 C. to about 30xc2x0 C. cooler than to average reactor temperature. Further, the average coke content of regenerated catalyst is less than about 2%, by weight, and the average coke content of the combination of separated and regenerated catalyst is from about 2%, by weight, to about 30%, by weight.
In another embodiment of the invention, a portion of the separated catalyst is regenerated and combined with the cooled portion of the catalyst prior to contacting the cooled portion of the catalyst with additional oxygenate-containing feedstock. Alternatively, a portion of the separated catalyst is regenerated and combined with the cooled portion of the catalyst after contacting the cooled portion of the catalyst with additional oxygenate-containing feedstock.
The invention further provides a method in which the olefin-containing product is separated from the catalyst and collected. The recovered olefin-containing product may contain at least about 85%, by weight, of ethylene, propylene, or mixtures thereof.
The invention further provides a method in which the oxygenate-containing feedstock may be selected from methanol, ethanol, n-propanol, isopropanol, C4-C20 alcohols, methyl ethyl ether, dimethyl ether, diethyl ether, di-isopropyl ether, formaldehyde, dimethyl carbonate, dimethyl ketone, acetic acid, and mixtures thereof.
The invention further provides that the molecular sieve catalyst is a silicoaluminophosphate molecular sieve selected from SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, metal containing forms thereof, mixtures thereof, and intergrowths thereof. The molecular sieve catalyst may be contacted with the oxygenate-containing feedstock at a temperature of from about 200xc2x0 C. to about 700xc2x0 C. In one embodiment, the molecular sieve catalyst is contacted with the oxygenate-containing feedstock at a gas superficial velocity of at least about 1.0 meters per second, and preferably at least about 2.0 meters per second.
In another embodiment, the invention provides polypropylene and/or polyethylene manufactured according to the method of the present invention.
The present invention will be better understood by reference to the Detailed Description of the Invention when taken together with the attached drawings.